1. Field of the Invention
The present invention relates to an aqueous ink, and particularly to an aqueous ink suitable for use in a recording method and a recording apparatus using an ink jet recording system.
2. Description of the Related Art
An ink used in ink jet recording systems is required to become an ink giving a high image density (OD value) for the purpose of achieving high image quality. In addition, an ink which prevents recording media from being stained even when recording is continuously conducted, i.e., an ink having excellent fixing ability, is required for the purpose of achieving speeding-up. Further, upon high-speed recording, white stripes (linked stripes) may occur at a boundary between images recorded through each one line recording of each scanning of a carriage in some cases to incur deterioration of image quality, and so improvement is required.
Various techniques have been proposed to date for the purpose of enhancing the image density. For example, the proposed techniques include a technique of using a coloring material excellent in color developing properties and a technique of properly designing the composition of an ink, thereby enhancing the association or aggregation ability of a coloring material to leave a larger amount of the coloring material on the surface of a recording medium.
It has heretofore been known that an ink containing a water-insoluble coloring material such as a pigment as a coloring material (pigment ink) provides images excellent in fastness properties such as water fastness and light fastness. In recent years, various techniques have been proposed for the purpose of more enhancing the image density of images formed with such an ink. For example, it has been proposed to use an ink containing self-dispersion type carbon black and a specific salt, thereby enhancing the image density (Japanese Patent Application Laid-Open No. 2000-198955; hereinafter referred to as Patent Document 1). In this technique, a pigment present in a dispersed state in the ink is forcedly aggregated on the surface of a recording medium, thereby inhibiting the permeation of the pigment into the recording medium to achieve a higher image density.
However, it has been found that since pigment particles are caused to aggregate on the surface of the recording medium in such prior art as mentioned above, an area of the surface of the recording medium which can be covered with the pigment with respect to the volume of ink droplets (the so-called area factor) may not be sufficient in some cases. This fact means that the amount of the ink applied, which is required to achieve the same image density, in the above-described technique increases compared with a conventional pigment ink containing a pigment dispersed by a dispersant. There has thus been room for improvement in this respect.
In addition, in such prior art as mentioned above, the amount of the ink applied is increased to require a long time for drying, so that in some cases, high-speed continuous recording may cause problems such as print-through upon the continuous recording as described below. More specifically, since the ink of a first recorded matter is not sufficiently dried (fixed) during the period from formation of the first recorded matter to discharge of a second recorded matter from an ink jet recording apparatus, the ink of the first recorded matter may adhere to the back surface of the second recording medium in some cases. Further, the above-described area factor may be insufficient in some cases for the requirement of high resolution in the ink jet recording apparatus. There has thus been room for improvement in this respect, too. Incidentally, there is a method for achieving a high area factor even with small volume ink droplets by improving the permeability of an ink into a recording medium. However, when the permeability of the ink is improved, the ink permeates not only in the surface of the recording medium, but also in the depth-wise direction of the recording medium, so that a sufficient image density may have not been achieved in some cases.
The following proposals have been made for solving such problems. For example, there is a proposal as to improvement in the image density by causing an ink containing a plurality of water-soluble organic solvents and a water-insoluble coloring material to have the following constitution, thereby aggregating a coloring material in the vicinity of the surface of a recording medium even with small volume ink droplets (Japanese Patent Application Laid-Open No. 2005-206615; hereinafter referred to as Patent Document 2). More specifically, the plurality of the water-soluble organic solvents includes a good medium and a poor medium for a self-dispersion type pigment, and a water-soluble organic solvent exhibiting a maximum Ka value among respective Ka values of the plurality of the water-soluble organic solvents as determined by the Bristow's method is a poor medium.
As techniques for controlling permeation of an ink into a recording medium and blurring of the ink, proposals paying attention to a dynamic surface tension of the ink have been made. For example, there has been a proposal to improve drying ability by an ink having the construction of [dynamic surface tension (dyn/cm) at a lifetime of 0 millisecond+viscosity (cp)]=42 to 49 (Japanese Patent Registration No. 2516218; hereinafter referred to as Patent Document 3). There is also a proposal as to an ink having a maximum value of the rate of change of the dynamic surface tension of from 0.2 mN/m/s or more to 0.4 mN/m/s or less (Japanese Patent Application Laid-Open No. 2003-238851; hereinafter referred to as Patent Document 4). There is further a proposal as to an ink having a dynamic surface tension of from 25 to 50 mN/m at a lifetime of 10 milliseconds and having a difference of 5 mN/m or less between the maximum value and the minimum value of the dynamic surface tension (Japanese Patent Application Laid-Open No. 2005-200566; hereinafter referred to as Patent Document 5). There is still further a proposal as to an ink having a dry viscosity of 100 mPa·s or less, a dynamic surface tension of 45 mN/m or less at a lifetime of 10 milliseconds and a dynamic surface tension of 35 mN/m at a lifetime of 1,000 milliseconds (Japanese Patent Application Laid-Open No. 2003-231838; hereinafter referred to as Patent Document 6).
In addition, there are the following proposals as to change in the particle size of a pigment and dispersion stability when a pigment ink containing water and a water-soluble organic solvent is evaporated. For example, there is a proposal that the increase rate of the average particle size of a pigment in a pigment ink when 30 mass % of the ink is evaporated is controlled to 25% or less (Japanese Patent Application Laid-Open No. 2002-167534; hereinafter referred to as Patent Document 7). There is also a proposal as to a pigment ink by which dispersion stability is achieved even when 50 mass % of the ink is evaporated (Japanese Patent Application Laid-Open No. 2004-143290; hereinafter referred to as Patent Document 8).